Day One

We can talk about triangles by describing (1) how many sides it has congruent (2) its angles.

Let's look at triangles that are named by the number of congruent sides. How many sides of a triangle could be congruent?


A triangle with all three sides congruent to each other is called an equilateral triangle.


A triangle with two of its sides congruent to one another is called an isosceles triangle. Click here to inevestigate the triangle shown. What is true about this triangle? Click here for a script to make your own isosceles triangle and here for a blank sketch pad. Test to see if your observations about the above triangle are also true for the isosceles triangles thatyou make?


What would a triangle look like that had only one side congruent. Review what it means for things to be congruent. Is it possible? Why or why not?


What does it look like for a triangle to have no sides congruent? Click here for a blank sketch pad to try and draw a few. These triangles are called scalene. The following is an example:

Now let's look at triangle that are named according to the types of angles they have.

All triangles have two acute angles? Why? (what does it mean for an angle to be acute? If an angle is not acute, what is it?) The third angle is used to classify the triangle. Review: what are the three classifications of angles: 90 degrees, acute--measures between zero and 90 degrees, obtuse--measures between 90 and 180 degrees.

"That's RIGHT," if the third angle is 90 degrees, a right angle, then the triangle is called a right triangle. In this case, why do the other two angles have to be acute?

How can you make a right triangle? Click here and try.

When the third angle is acute, the triangle is "A-CUTE"one.

When the third angle is obtuse, the triangle is also called an obtuse triangle.

If one angle is obtuse, why must the other two be acute?

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